add xai model client

[me020523]

Why are these changes needed?

enable autogen to support XAI models

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[me020523]

@microsoft-github-policy-service agree

[ekzhu]

Thanks for the PR. Generally, for model clients we encourage you to create and maintain a community Pypi package, and we can index them in our documentation.

See: https://microsoft.github.io/autogen/dev/user-guide/extensions-user-guide/index.html

[me020523]

Thank you for your response and suggestion.

[ekzhu]

@me020523 you can update this PR to just adding an entry that points to your package once you have created it.